This invention relates to the discovery that removal of carbonates and bicarbonates from a tailings material effects more rapid settlement of solids in the tailings sludge suspension.
In general, these sludge suspensions are aqueous colloidal suspensions containing either clay minerals or metal oxides-hydroxides which are formed in large volume during mining operations in the recovery of such materials as coal, bitumen from tar sands, and metals. In the case of metal mining operations, suspensions known as slimes are formed, typically phosphate slimes or like materials produced in the mining of copper, nickel, and titanium. In coal and tar sands minings, for example, the mining effluent typically contains dilute or thick clay mineral suspensions.
In order to properly dispose of these voluminous mining discharges, regardless of their source, flocculants have conventionally been employed to destablize these suspensions and thus permit the effective separation of water from solids.
This invention relates to the treatment of tailing pond water obtained from the hot water process for treating bituminous sands, such as Athabasca tar sands, and, more particularly, to the treatment of the water and clay-containing effluent discharged from the process.
Tar sands (which are also known as oil sands and bituminous sands) are sand deposits which are impregnated with dense, viscous petroleum. Tar sands are found throughout the world, often in the same geographical area as conventional petroleum. The largest deposit, and the only one of present commercial importance, is in the Athabasca area in the northeast of the Province of Alberta, Canada. This deposit is believed to contain over 700 billion barrels of bitumen. For comparison, this is just equal to the world-wide reserves of conventional oil, 60% of which is found in the middle east.
Athabasca tar sand in a three-component mixture of bitumen, mineral and water. Bitumen is the material for which tar sands are mined and processed. The bitumen content is variable averaging 12 wt.% of the deposit, but ranging from 0 to 18 wt.%. Water typically runs 3 to 6 wt.% of the mixture, increasing as bitumen content decreases. The mineral content is relatively constant ranging from 84 to 86 wt.%.
Several basic extraction methods have been known for many years for separating the bitumen from the sands. In the so-called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of a neutral salt in water. The combined mass is then subjected to a pressure or gravity separation.
The hot water process for primary extraction of bitumen from tar sands consists of three major process steps and a fourth step, final extraction, is used to clean up the recovered bitumen for downstream processing. In the first step, called conditioning, tar sand is mixed with water and heated with open steam to form a pulp of 70 to 85 wt.% solids. Sodium hydroxide or other reagents are added as required to maintain pH in the range of 8.0-8.5. In the second step, called separation, the conditioned pulp is diluted further so that settling can take place. The bulk of the sand-size mineral rapidly settles and is withdrawn as sand tailings. Most of the bitumen rapidly floats (settles upward) to form a coherent mass known as froth which is recovered by skimming the settling vessel. A third stream may be withdrawn from the settling vessel. This stream, called the middlings drag stream, may be subjected to a third processing step, scavenging. This step provides incremental recovery of suspended bitumen and can be accomplished by conventional froth flotation.
The mineral particle size distribution is particularly significant to operation of the hot water process and to sludge accumulation. The terms sand, silt, clay, and fines are used in this specification as particle size designations wherein sand is siliceous material which will not pass a 325 mesh screen. Silt will pass 325 mesh, but is larger than 2 microns, and clay is material smaller than 2 microns including some siliceous material of that size.
Conditioning tar sands for the recovery of bitumen consists of heating the tar sand/water feed mixture to process temperature (180.degree.-200.degree. F.), physically mixing the pulp to uniform composition and consistency, and the consumption (by chemical reaction) of the caustic or other reagents added. Under these conditions, bitumen is stripped from the individual sand grains and mixed into the pulp in the form of discrete droplets of a particle size on the same order as that of the sand grains. The same process conditions, it turns out, are also ideal for accomplishing deflocculation of the clays which occur naturally in the tar sand feed. Deflocculation, or dispersion, means breaking down the naturally occurring aggregates of clay particles to produce a slurry of individual particles. Thus, during conditioning, a large fraction of the clay particles become well dispersed and mixed throughout the pulp.
Those skilled in the art will therefore understand that the conditioning process, which prepares the bitumen for efficient recovery during the subsequent process steps also cause the clays to be most difficult to deal with in the tailings disposal operations.
The second process step, called separation, is actually the bitumen recovery step (the separation having already occurred during conditioning). The conditioned tar sand pulp is screened to remove rocks and unconditionable lumps of tar sands and clay. The reject material "screen oversize," is discarded. The screened pulp is further diluted with water to promote two settling processes: globules of bitumen, essentially mineral-free, settle (float) upward to form a coherent mass of froth on the surface of the separation cells; and, at the same time, mineral particles, particularly the sand size mineral, settle down and are removed from the bottom of the separation cell as tailings. The medium through which these two settling processes take place is called the middlings. Middlings consist primarily of water, with suspended fine material and bitumen particles.
The particle sizes and densities of the sand and of the bitumen particles are relatively fixed. The parameter which influences the settling processes most is the viscosity of the middlings. Characteristically, as the fines content rises above a certain threshold (which varies according to the composition of the fines), viscosity rapidly achieves high values with the effect that the settling processes essentially stop. In this operating condition, the separation cell is said to be "upset." Little or no oil is recovered, and all streams existing the cell have about the same composition as the feed.
As feed fines content increases, more water must be used in the process to maintain middlings viscosity within the operable range. For most feeds, over a wide range of fines contents, a clay-water ratio of approximately 0.1 represents the upper limit of operability.
The third step of the hot water process is scavenging. The feed fines content sets the process water requirement through the need to control middlings viscosity which, as noted above, is governed by the clay/water ratio. It is usually necessary to withdraw a drag stream of middlings to maintain the separation cell material balance, and this stream of middlings can be scavenged for recovery of incremental amounts of bitumen. Air flotation is an effective scavenging method for this middlings stream.
Final extraction or froth clean-up is usually accomplished by centrifugation. Froth from primary extraction is diluted with naptha, and the diluted froth is then subjected to a two stage centrifugation. This process yields an oil product of an essentially pure (diluted) bitumen. Water and mineral removed from the froth constitute an additional tailing stream which must be disposed of.
In the terminology of extractive processing, tailings is the throwaway material generated in the course of extracting the valuable material from an ore. In tar sands processing, tailings consist of the whole tar sand ore body plus net additions of process water less only the recovered bitumen product. Tar sand tailings can be subdivided into three categories; vis: (1) screen oversize, (2) sand tailings (the fraction that settles rapidly), and (3) tailings sludge (the fraction that settles slowly). Screen oversize is typically collected and handled as a separate stream.
Tailings disposal includes all of the operations required to place the tailings in a final resting place. One obvious long-range goal of tailings disposal is to return the tailings to the mined out area in a satisfactory form. Thus, there are two main operating modes for tailings disposal: (1) dike building which involves hydraulic conveying of tailings followed by mechanical compaction of the sand tailings fraction; and (2) overboarding which involves hydraulic transport with no mechanical compaction.
Recently, in view of the high level of ecological consciousness in Canada and the United States, technical interest in tar sands operation has begun to focus on tailings disposal. The concept of tar sands tailings disposal is straightforward. Visualize mining one cubic foot of tar sands which leaves a one cubic foot hole in the ground. The ore is processed to recover the resource (bitumen) and the remainder, including both process material and the gangue constitutes the tailings which are not valuable and are to be disposed of. In tar sands processing, the main process material in water and the gangue is mostly sand with some silt and clay. Physically, the tailings consists of a solid part (sand tailings) and a more or less fluid party (sludge). The most satisfactory place to dispose of these tailings is, of course, the existing one cubic foot hole in the ground. It turns out, however, that the sand tailings alone from the one cubic foot of ore occupy just about one cubic foot. The additional amount of sludge is variable, depending on ore quality and process conditions, but may run up to 0.3 cubic feet. Thus, the tailings simply will not fit back into the original hole in the ground.
This historical literature covering the hot water process for the recovery of bitumen from tar sands contains little in the way of a recognition that a net accumulation of liquid tailings or sludge would occur. Based on analysis of field test unit operations which led to the oil sands plant design near Ft. McMurray, Alberta, the existence of sludge accumulation was predicted. This accumulation came to be called the "pond water problem." Observations during start-up and early commercial operations at Ft. McMurray (1967-69) were of insufficient precision to confirm the prediction. Since 1969, commercial operating data have confirmed the accumulation in the tailings disposal area of a layer of fine material and water (sludge) which settles and compacts only very slowly, if at all, after a few years.
For dike building at the tar sands plant tailings are conveyed hydraulically to the disposal area and discharged onto the top of a sand dike which is constructed to serve as an impoundment for a pool of liquid contained inside. On the dike, sand settles rapidly, and a slurry of fines, water, and minor amounts of bitumen flows into the pond interior. The settled sand is mechanically compacted to build the dike to a higher level. The slurry which drains into the pond interior commences stratification after an induction period of about a month or more and settling occurs over a time scale of months to years. As a result of this long-term settling, two layers form. The top 5 to 10 feet of the pool are a layer of relatively clear water containing 0 to 5 wt.% solids. Below this clear water layer is a discontinuity in solids content. Over a matter of a few feet, solids content increases to 10-15 wt.%, and thereafter, solids content increases regularly toward the pond bottom. In the deepest parts of the pond, solid contents of over 50 wt.% have been recorded. This second layer is called the sludge layer. The solids content of the sludge layer increases regularly from top to bottom by a factor of 4-5. The clay-water ratio in this layer increases also, but by a lower factor 1.5-2.5. The clays, dispersed during processing, apparently have partially reflocculated into a very fragile gel network. Through this gel, fines of larger-than-clay sizes are slowly settling.
Overboarding is the operation in which tailings are discharged over the top of the sand dike directly into the liquid pool. A rapid and slow settling process occur, but their distinction is not as sharp as in dike building and no mechanical compaction is carried out. The sand portion of the tailings settles rapidly to form a gently sloping beach extending from the discharge point toward the pond interior. As the sand settles, fines and water drain into the pool and commence long-term settling.
In summary: (1) tar sands contain clay minerals, (2) in the hot water extraction process, most of the clays become dispersed in the process streams and traverse the circuit, exiting in the tailings, (3) the amount of process water input is fixed by the clay content of the feed and the need to control viscosity of the middlings stream, (4) the amount of water required for middlings viscosity control represents a large volume relative to the volume of the ore itself, and (5) upon disposal, clays settle only very very slowly; thus, the process water component of tailings is only partially available for reuse via recycle. That which can't be recycled represents a net accumulation of tailings sludge.
Thus, to alleviate the pond water problem it is necessary to devise long-term economically and ecologically acceptable means to eliminate, minimize, or permanently dispose of, the accumulation of liquid tailings or sludge.
Flocculation of the drag stream in order to improve the settling characteristics thereto has been proposed and practiced in the prior art. In flocculation, individual particles (in this case clay particles) are united into rather loosely bound agglomerates or flocs. The degree of flocculation is controlled by the probability of collisions between the clay particles and their tendency toward adhesion after collision. Agitation increases the probability of collision and adhesion tendency is increased by the addition of flocculants.
Reagents act as flocculants through one or a combination of three general mechanisms: (1) neutralization of the electrical repulsive forces surrounding the small particles which enables the van der Waals cohesive force to hold the particles together once they have collided; (2) precipitation of voluminous flocs, such as metal hydroxides, that entrap fine particles; and (3) bridging of particles by natural or synthetic, long-chain, high-molecular-weight polymers. These polyelectrolytes are believed to act by absorption (by ester formation or hydrogen bonding) or hydroxyl or amide groups on solid surfaces, each polymer chain bridging between more than on solid particle in the suspension.
Among the various reagents which have been found useful for flocculating clay are: alumimum chloride, polyalkylene oxides, such as polyethylene oxide, compounds or calcium such as calcium hydroxide, calcium oxide, calcium chloride, calcium nitrate, calcium acid phosphate, calcium sulfate, calcium tartrate, calcium citrate, calcium sulfonate, calcium lactate, the calcium salt of ethylene diamine tetraacetate and similar organic sequestering agents. Also useful are quartz flour or a high molecular weight acrylamide polymer such as polyacrylamide or a copolymer or acrylamide and a copolymerizable carboxylic acid such as acrylic acid. Additional flocculants which have been considered include the polymers of acrylic or methacrylic acid derivatives, for example, acrylic acid, methacrylic acid, the alkali metal and ammonium salts of acrylic acid or methacrylic acid, acrylamide, methacrylamide, the aminoalkyl acrylates, the aminoalkyl methacrylamides and the N-alkyl substituted aminoalkyl esters of either acrylic or methacrylic acids.
Those skilled in the art will understand that a satisfactory solution to the "pond water problem" must be economically, as well as ecologically acceptable. A distinct step forward in the art was achieved by the use of hyrdolyzed corn and potato starch flocculants as set forth in copending U.S. application Ser. No. 145,296, now issued as U.S. Pat. No. 4,330,409 entitled Destabilization of Sludges with Hydrolyzed Starch Flocculants. The disclosure of this patent, which is hereby incorporated by reference, points out the advantages of using as a flocculant a hydrolyzed wheat, corn or potato starch obtained by the aqueous hydrolysis of the starch in the presence of one or more insoluble metal salts formed in situ and such flocculants will be used in conjunction with this invention.